This invention relates in general to refrigeration circuits, and, in particular, to a defrost system for a refrigeration circuit such as may be incorporated in air conditioning apparatus including a heat pump.
More specifically, but without restriction to the particular use which is shown and described, this invention relates to a hot gas defrost system wherein a portion of the superheated gaseous heat transfer fluid is conducted through a heat exchanger to melt accumulated ice from the heat exchanger thereby causing the heat transfer fluid passing within the heat exchanger to condense. The condensed heat transfer fluid is passed through a suction line to an accumulator into which a portion of the superheated gaseous heat transfer fluid is conducted. The gaseous superheated refrigerant acts to vaporize the condensed heat transfer fluid contained within the accumulator to provide a supply of gaseous refrigerant to the compressor inlet.
Heat pumps, for example, function to transfer heat between an indoor coil and an outdoor coil through the use of a heat-exchange fluid which is selectively vaporized and condensed in accordance with the desired mode of operation. During warm weather, warm air indoors is circulated about an indoor coil so that the heat from the indoor air is absorbed by the heat-exchange fluid, or referigerant, which is then carried outdoors to the outdoor coil releasing the heat to the surrounding air. In cold weather the cycle is reversed. Heat, which has already been produced outdoors by the sun and stored in the earth and air, is transferred to the heat-exchange fluid by the outdoor coil and discharged from the heat-exchange fluid indoors.
One of the frequently encountered and well known problems associated with such heat pump equipment is that during heating operations the outdoor coil, which is functioning as an evaporator, tends to accumulate frost or ice when the appropriate weather conditions occur. The accumulation of frost on the outdoor coil reduces the ability of the heat exchanger to transfer heat from the ambient air in contact with the heat exchanger surfaces to the refrigerant. In order to remove the accumulated frost and ice from the surfaces of the outdoor coil, various automatic defrosting systems have been devised. These systems include heating the coil from an external heat source, and reversing the operation of the system to pass hot refrigerant gas through the outdoor coil.
In such hot gas reverse defrost systems, the gas conducted to the outdoor coil melts the ice formed thereon and thereby changes state from a gas to a liquid within the outdoor coil. The condensed heat-exchange fluid, or liquid refrigerant, is then flashed to a gas in an evaporator and any remaining liquid refrigerant is collected in an accumulator which separates and retains liquid refrigerant to prevent the liquid from being conveyed into and damaging the system compressor. Liquid refrigerant in the compressor is called "slugging" and may result in physical damage to the compressor components.
While such systems are satisfactory for removing the accumulation of ice from the outdoor coil, and preventing the liquid refrigerant from damaging the system compressor, the reversing process however causes high system stress and high system noise level during abrupt system reversals. Such systems usually require a second heat source, usually electric resistance heat to replace the heat removed from the indoor space by the indoor coil during defrost. In addition, as liquid refrigerant is accumulated in the accumulator, provision must be made to remove the accumulation, such as by re-introducing a controlled amount of the liquid into the gaseous refrigerant as a fine suspension through an accumulator.
In the present hot gas defrost system, superheated defrosting gas is by-passed around the expansion valve and discharged from the compressor outlet directly into the inlet of the outdoor coil wherein the hot gas is condensed melting the ice and liquid refrigerant or a mixture of gaseous and liquid refrigerant is discharged from the outdoor coil outlet. The outlet from the outdoor coil is coupled to a component which serves as an accumulator to prevent the liquid refrigerant from being introduced into the system compressor and as a re-evaporator to vaporize the liquid refrigerant received from the compressor. A portion of the superheated hot gas by-passes the indoor coil and expansion valve and is introduced directly into the liquid refrigerant contained in the accumulator to vaporize the liquid refrigerant. The vaporized liquid refrigerant is thereby returned for use by the system compressor providing a more efficient system to control defrost operation. The heat energy of this defrost system may be constantly monitored to determine when defrost has been effected. The accumulator serves as a receiver for storing liquid refrigerant during heat transfer operation and as a direct contact heat exchanger during defrost operations.